The present invention relates to a flow control system and method for controlling the batchwise delivery of process gas to a semiconductor manufacturing tool. Functional components of the system are assembled on a gas manifold in the form of a narrow delivery xe2x80x9cstickxe2x80x9d.
The semiconductor manufacturing process includes a phase in which the process gas is delivered to the tool according to a program that specifies a flow for a period of time. The flow rate is established by a mass flow controller, which is supplied with process gas at a regulated pressure. The output of the mass flow controller is delivered to the manufacturing tool through a pneumatically operated on/off shut-off valve. The delivery is started by actuating the shut-off valve to open and energizing the mass flow controller to deliver a flow at a preset value. The delivery is stopped by actuating the shut-off valve to close and de-energizing the set point of the mass flow controller.
An important consideration is the accuracy with which the flow is delivered during the process phase. To that effect, it is recommended to set the mass flow controller at a value between 40 and 100% of its full scale. In other words, a given mass flow controller has a rangeability of 2.5 to 1. Also, the mass flow controller must be calibrated for the specific gas to which it is applied. This means that in order to cover a range of flows from 5 to 1000 standard cubic centimeters per minute (sccm), as many as six mass flow controllers for any given gas may be required. The accuracy consideration also requires that the mass flow controller retain its calibration for some period of time, preferably as long as possible.
A dynamic gas flow controller is disclosed in U.S. Pat. No. 5,865,205 to Wilmer, for controlling the delivery of a gas from a reservoir to a semiconductor process chamber. The method and apparatus disclosed therein involve determining an initial mass of the gas residing within the reservoir prior to a delivery operation and the final mass of gas residing in the reservoir when the flow of gas to the process chamber is terminated. The initial mass and final mass of gas values are compared to determine the actual mass of gas released from the reservoir during the recipe step. This value serves as an input to a calibration servo loop to update the system calibration constant for a subsequent gas delivery recipe step. The execution of the calibration servo loop serves as a continuous self calibration of a dynamic servo loop, wherein the flow of gas to the process chamber is metered by a variable flow valve upstream of an orifice. The gas pressure created ahead of the orifice during delivery is sensed to measure the gas flow rate.
In the patent to Wilmer, the concept of flow control applies to a gas flowing out of a reservoir instead of a gas flowing in a line. The control signal which operates the variable flow valve is determined by a circuit in which an input signal (representing the desired flow) is integrated over the duration of the delivery step to define the desired volume/mass. The desired volume is compared to the actual volume taken out of the reservoir. A control signal is generated as a function of that comparison and applied as a set point to the control circuit. In the dynamic control circuit, the set point is compared to the pressure signal sensing the flow and a control signal is applied to the variable flow valve to create the desired pressure/flow. That is, in Wilmer, the flow signal is integrated over time and compared to the actual volume. The control signal is applied to the flow control, which consists of the variable flow valve creating a pressure ahead of the orifice.
The patent to Kennedy, U.S. Pat. No. 4,285,245, discloses a method and apparatus for measuring and controlling volumetric flow rate of gases in a line. The patent is of interest for its disclosure of a method of determining the flow rate of gas flowing in a line by imposing a uniform flow rate at a point downstream of a flow measurement chamber in the line, restricting temporarily the flow at a point upstream of the chamber and measuring the pressure decrease in the chamber between the upstream and the downstream points during part of the duration of the restricted flow, the rate of the pressure decrease being substantially proportional to the volumetric flow rate. The patent to Kennedy does not relate to the batchwise delivery of process gas for semiconductor manufacturing or a flow control system therefor operable in a flow mode for the accurate delivery of a batch of process gas and, alternately, in a no-flow mode.
There is a need for an improved method and flow control system for controlling the batchwise delivery of process gas for semiconductor manufacturing, which can complement a conventional delivery stick by the addition of only a few components, and which will allow verification of the accuracy of the flow delivered in an active phase. There is also a need for an improved method and flow control system for controlling the batchwise delivery of process gas for semiconductor manufacturing which increase the effective range of the mass flow controller, ensure long-term stability of the calibration and eliminate the need to pre-calibrate for specific gases.
The method of the invention for controlling the batchwise delivery of process gas for semiconductor manufacturing using a flow control system of the invention operable in a flow mode for delivery of a batch of process gas and, alternately, in a no-flow mode, comprises delivering a batch of process gas from a source of pressurized process gas through a flow line of the flow control system to a semiconductor manufacturing apparatus at a controlled flow rate for a delivery period of time. The line of the flow control system includes a pressure regulator for establishing a regulated pressure of the process gas in the line, an on/off valve downstream of the pressure regulator to start and stop the flow mode during which the process gas is delivered to the apparatus for the delivery period of time and, upstream in the line from the pressure regulator, a reference capacity used to measure the actual flow rate of the delivery.
The method further comprises, after the start of the delivering of the batch of process gas, measuring for a measurement period of time the pressure drop of the process gas in the reference capacity while interrupting the flow of process gas through the line to the referenced capacity and continuing to deliver process gas from the line of the flow control system to the semiconductor manufacturing apparatus at the controlled flow rate. The rate of pressure drop in the reference capacity during the measurement period and the actual flow rate of the batch of process gas being delivered are determined from the measuring. In case the actual flow rate does not agree with a specified flow rate for the delivering, the controlled flow rate is adjusted in the direction of the specified flow rate from the actual flow rate for a subsequent delivery period of time in which another batch of process gas is delivered.
A flow control system according to the invention is for use within a fluid circuit having a source of pressurized gas to be delivered batchwise at a controlled flow rate to a destination by the flow control system. The flow control system is operable in a flow mode for delivering a batch of gas and, alternately, in a no-flow mode. The flow control system comprises a flow line through which gas from the source of pressurized gas can be delivered, a pressure regulator in the flow line to establish a regulated gas pressure in the line, an on/off valve in the line downstream of the pressure regulator to start and stop the flow mode during which the gas is delivered for a delivery period of time, a reference capacity in the flow line upstream of the pressure regulator for use in measuring the actual flow rate of gas being delivered by the flow control system, a pressure sensor to measure a pressure drop of the gas in the reference capacity during a measurement period of time commencing after the start of a delivery period of time, means in the line upstream of the reference capacity for interrupting the flow of the gas from the source of pressurized gas to the reference capacity during delivery of the gas by the flow control system, and a controller for determining from the measured pressure drop the rate of pressure drop in the reference capacity during the measurement period the actual flow rate of a batch of process gas being delivered and, in case the actual flow rate does not agree with a specified flow rate for the delivering, adjusting the controlled flow rate in the direction of the specified flow rate from the actual flow rate for a subsequent delivery period of time in which another batch of process gas is delivered.
In one embodiment of the invention, the flow control arrangement of the system comprises a mass flow control valve in the line downstream of the pressure regulator. The controller of the system adjusts a set point value of the mass flow control valve for adjusting the controlled flow rate. Advantageously, the mass flow control valve has a range of possible controlled flow rate settings which extends to 100% of its full scale with an effective rangeability of 10:1.
According to another form of the invention, the flow control arrangement of the system comprises a fixed orifice in the flow line downstream of the pressure regulator. The pressure regulator has an adjustable pressure setting for adjusting the controlled flow rate.
The flow control system of the invention uses functional components compatible with a 1xe2x85x9 inch width manifold. The assembly features surface mounting on a modular base. A significant benefit is to reduce the length of the delivery stick and to make it possible to place the parallel manifolds side-by-side at a distance of 1.2 inches between center lines instead of the current 1.6 inches. In the flow control system employing a mass flow control valve, increasing the effective rangeability of the controller from 2.5 to 1 up to 10 to 1, makes it possible to cover flows from 5 to 1000 sccm with only three ranges: 1000, 200 and 50 sccm. The long term stability of calibration is also ensured through automatic calibration during each active phase and the need to calibrate for each specific gas is eliminated.
These and other features and advantages of the present invention will become more apparent from the following detailed description of several embodiments of the present invention taken with the accompanying drawings.